add example config for sysmobts

Many years ago, there was no difference between the libbsc support for
nanobts and sysmobts.  However, this is not the case for a long time
anymore, and there are some specifics in OsmoNITB when it comes to
sysmobts.  Let's have an example config file

Change-Id: I94ae57c9a3cb497ca39d56270fa15ed65d7f147e

openbsc/doc/examples/osmo-nitb/sysmobts/openbsc.cfg

@@ -0,0 +1,77 @@
+!
+! OpenBSC configuration saved from vty
+!   !
+password foo
+!
+line vty
+ no login
+!
+e1_input
+ e1_line 0 driver ipa
+network
+ network country code 1
+ mobile network code 1
+ short name OpenBSC
+ long name OpenBSC
+ auth policy closed
+ location updating reject cause 13
+ encryption a5 0
+ neci 1
+ rrlp mode none
+ mm info 1
+ handover 0
+ handover window rxlev averaging 10
+ handover window rxqual averaging 1
+ handover window rxlev neighbor averaging 10
+ handover power budget interval 6
+ handover power budget hysteresis 3
+ handover maximum distance 9999
+ timer t3101 10
+ timer t3103 0
+ timer t3105 0
+ timer t3107 0
+ timer t3109 4
+ timer t3111 0
+ timer t3113 60
+ timer t3115 0
+ timer t3117 0
+ timer t3119 0
+ timer t3141 0
+ bts 0
+  type sysmobts
+  band DCS1800
+  cell_identity 0
+  location_area_code 1
+  training_sequence_code 7
+  base_station_id_code 63
+  ms max power 15
+  cell reselection hysteresis 4
+  rxlev access min 0
+  channel allocator ascending
+  rach tx integer 9
+  rach max transmission 7
+  ip.access unit_id 1801 0
+  oml ip.access stream_id 255 line 0
+  gprs mode none
+  trx 0
+   rf_locked 0
+   arfcn 514
+   nominal power 23
+   max_power_red 20
+   rsl e1 tei 0
+   timeslot 0
+    phys_chan_config CCCH+SDCCH4
+   timeslot 1
+    phys_chan_config SDCCH8
+   timeslot 2
+    phys_chan_config TCH/F
+   timeslot 3
+    phys_chan_config TCH/F
+   timeslot 4
+    phys_chan_config TCH/F
+   timeslot 5
+    phys_chan_config TCH/F
+   timeslot 6
+    phys_chan_config TCH/F
+   timeslot 7
+    phys_chan_config TCH/F